Prior studies have found distinctive alterations of brain event-related potentials (ERPs) in obsessive-compulsive disorder (OCD), e.g., reduced N200/P300 latency and enhanced negativities. Our long range goal is to help clarify the nature of the CNS mechanisms that may be different in OCD by assessing differences between OCD patients and controls in endogenous ERP components, each of which is thought to reflect a specific stage of information processing. Passive and active attention tasks will be used to identify the specific components within the N200- P300 complex that are altered in OCD patients. Tasks designed to measure attention-related processing negativity (Nd) and the contingent negative variation (CNV) will test hypotheses concerning the existence of overfocused attention and excessive doubt and uncertainty in OCD. Also, relationships between lateral asymmetries of ERP components and cognitive asymmetries for a dichotic syllable task will provide data for evaluating hypotheses of left hemisphere overactivation and dysfunction in OCD. The proposed project will also investigate: a) the diagnostic specificity of ERP findings for OCD by using a social phobic control group; b) relationships between ERP abnormalities and symptom type and severity in OCD; and c) whether key ERP abnormalities in OCD represent stable (trait) characteristics or change following successful treatment with an antiobsessional medication. Specifically, 40 OCD patients, 20 social phobic controls and 20 normal controls will be tested on four ERP tasks; a Passive Attention Task, an Active Attention Task, a Directed Attention Task and a Choice Reaction Time CNV Task, as well as on a dichotic consonant-vowel discrimination task. OCD patients and social phobics are tested after a minimum drug- free period of 14 days, and OCD patients are retested following 12 weeks of treatment with an antiobsessional mediation, fluoxetine in most cases. Normal controls will be retested at the same test-retest intervals as OCD patients.